1. Field of the Invention
The present invention relates to a telephone system, and more particularly, to a method for controlling an inter-frequency handoff in a code division multiple access (CDMA) cellular telephone system.
2. Background of the Related Art
Generally, a cellular telephone system includes a service area which is divided into a plurality of cells. Each cell provides many mobile terminals with a radio communication service through a base station. To maintain high quality communication when the mobile terminal moves between cells, a handoff, which changes a communication link or channel with one base station to another base station, must be timely established. Therefore, the mobile terminal has to be able to measure the quality of signals from other peripheral base stations during communicatiom, and the base station system is provided with the quality of signal information to timely perform the handoff. Such a hand off is referred to as a mobile-assisted handoff.
In particular, the CDMA system can perform the communication while using the same frequency and varying an offset of a common pseudo-random code that spreads bandwidth of transmitting signal. The CDMA system has a first digital receiver, which is referred to a searcher for searching a different CDMA signal of the same frequency, and a second digital receiver for demodulating the data.
Typically, the CDMA system uses a plurality of second digital receivers, which are referred to as xe2x80x9cRAKExe2x80x9d receiver, to receive the data transmitted via multiple paths. Accordingly, the mobile terminal can measure the quality of a different base station signal of the same frequency during the communication. In the CDMA method, the transmission rate of the data can be varied in multilevel, and the overall radio capacity within the transmission frequency can be increased by reducing an interference between different channels, which use the same frequency as the transmission rate is reduced. For example, voice data has an information amount variable with the time, and half of the communication is used for listening, i.e., receiving voice data, and the rest is used for speaking, i.e., transmitting voice data. If a variable rate transmission is used, a fifty percent or more increase in capacity may be produced
The system capacity in the cellular telephone system may be increased by reducing the size of the cell to increase the number of the cells within the service area. However, if the size of the cell is reduced, the handoff is too often occurred generated when moving at a high speed. Hence, the base station has difficulty in properly processing the handoff.
Accordingly, a hierarchical cell structure or a layered cell structure is used, where a macro-cell of a larger size is placed upon a micro-cell of smaller size which serves the terminal moving at high-speed, is placed. In this cell structure, the handoff between the micro-cell and macro-cell according to a moving speed of the terminal, as well as the handoff between macro-cells, has to be provided.
In the hierarchical cell structure, the signal strength of the base station of the macro-cell may be much larger than that of the base station of the micro-cell, and therefore various problems, such as cocktail party effect, egg phenomenon, etc., may occur even in the CDMA system. To provide the hierarchical cell structure on different frequencies, in the CDMA system has to provide a (mobile-assisted) inter-frequency handoff.
To perform the (mobile-assisted) inter-frequency mobile-assisted handoff, the mobile terminal has to be able to receive the base station signal of different frequency during the communication and therefore, it has to be provided with an additional radio frequency RF receiver, or alternatively, the mobile terminal has to use a single RF receiver in a time-division multiplexing (TDM) method for receiving the signal of a different frequency.
The above-mentioned related CDMA cellular telephone system continuously transmits a forward signal from the base station toward the mobile terminal, the mobile terminal may have two or more RF receivers for receiving the base station signal of different frequencies during the communication. However, this method is undesirable since the cost and size of the mobile terminal increase.
For the alternate method in the CDMA system, the time period where the mobile terminal breaks a part of reception signal and receives the signal of the different frequency may be used for providing the inter-frequency handoff with a single RF receiver. However, such a method may cause the degradation of the communication quality and the communication may be dropped as the required signaling data is not received.
Accordingly, the present invention is directed to substantially obviate the problems of the related art.
An object of the present invention is to provide a method for providing inter-frequency handoff in a code division multiple access cellular telephone system capable of maintaining an excellent communication quality.
Another object of the present invention is to rapidly establish an inter-frequency handoff between a mobile terminal and a peripheral base station as the mobile terminal searches a frequency of the other base station or transmits searched information.
A further object of the present invention is to allow inter-frequency handoff without using dual transceivers.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for providing an inter-frequency handoff in a code division multiple access cellular telephone system comprises the steps of: forming a non-transmitting period in a transmission frame by controlling the repeat times of a transmission symbol in a frame and a position or form of a transmitting period with a variable rate characteristic of a transmission signal; controlling an energy transmission level of the signal reconstructed through the aforesaid step in inverse proportion to the repeat times to transmit it; and searching a base station signal of a different frequency through a period where a signal of the frame transmitted through the aforesaid step is not transmitted, or transmitting a signal to the base station of the different frequency, by a mobile terminal.
The present invention can be achieved in parts or in a whole by a method of performing a handoff in a telephone system comprising the steps of: modulating data to a prescribed energy level of a frame unit by controlling repeat times of a symbol; inserting a searching period into the modulated frame to reconstruct the modulated frame as a transmission frame; and searching a frequency information of a station with the transmission frame to perform a handoff.
The present invention can be achieved in parts or in a whole by a method for providing a handoff in a telephone system comprising the steps of: modulating a band into a modulated frame having a repeated frame period by varying an energy transmission level based on a full rate frame; forming a non-transmitting period to the modulated frame and reconstructing a repeat transmission frame by controlling a number of repetition of the energy level; and searching frequency information of a peripheral base station with the transmission frame to perform a handoff and transmitting the searched information to a transmitter of a base station.
The present invention can be achieved in parts or in a whole by a method for providing a handoff in a telephone system comprising the steps of: forming a non-transmitting period in a transmission frame by controlling a number of repetitions of a transmission symbol in a frame and at least one of a position and form of a transmitting period with a variable rate characteristic of the transmission frame; controlling an energy level of the transmission symbol by an inverse proportion of the number of repetitions; and searching a base station signal of a different frequency during the non-transmitting period; or transmitting a signal to the base station of the different frequency, by a mobile terminal.
The present invention can be achieved in parts or in a whole by a telephone system comprising: a station that modulates data into a transmission frame having an energy level based on an inverse proportion of a number of repetitions of a transmission symbol, and at least one of a position and form of a transmitting period with a variable rate characteristic of the transmission frame, the station forming a non-transmitting period based on the number of repetitions as a searching period; and a terminal that searches frequency information during the searching period and transmits the searched frequency information to the station.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.